Merge pull request #6351 from haskell/travis-xenial

Change travis dist: trusty -> xenial

.travis.yml

@@ -2,7 +2,7 @@
 # We specify language: c, so it doesn't default to e.g. ruby
 language: c
 
-dist: trusty
+dist: xenial
 
 # This sets the default config for each job to use full VMs.
 # The VMs have 2 cores and 8 gigs of ram. Larger VMs are also available.